Ferrite materials, such as manganese-zinc ferrite compounds, have been widely used as magnetic core materials for transformers in power supply systems, as well as for household electric appliances, communication and telecommunication equipment, computer and peripheral equipment, electronics finished products, electronic components, and other products that employ high frequency electronic circuitry. Ferrite materials have been found to exhibit properties such as, high permeability, high saturation magnetic flux density, high temperature stability, and low power losses that make these materials suitable for high frequency applications. For example, in transformer applications for power supply systems, sintered ferrite materials provide relatively low power losses and high temperature stability when used at relatively high switching frequencies. Typically, with switching frequencies ranging from 100 kHz to 500 kHz, power losses are measured to be about 300 milliwatts per cubic centimeter (mW/cm3) or greater, and Curie temperatures range from 230° C. to 240° C. As used herein, Curie temperature refers to the critical temperature at which ferrite materials substantially lose their magnetic characteristics. It is the combined benefits of relatively low power losses and high temperature stability, for example, that make ferrite compositions particularly well suited for various and wide ranging high frequency electrical applications.
Due, in part, to the increased demand for employing high frequency electronic circuitry into a wide range of components and equipment, efforts have been made to advance the ways in which power supplies can be improved and/or miniaturized for integration into these applications. This demand, at times, has been tempered by the premium that is placed on the available space inside these components. Typically, these efforts are directed to improving the ability of the power supply to perform at high temperatures and high frequencies with low core power losses, so that the size can be reduced without sacrificing performance or operation. Thus, much of the attention devoted to the miniaturization process is related to improvements to the material properties of the ferrite materials, and is based on the equation P˜fBA, wherein throughput power (P) is proportional to operating frequency (f), magnetic flux density (B), and magnetic cross section (A). Accordingly, increases in operating frequency and/or magnetic flux density allow for reductions in magnetic cross section without sacrificing throughput power. However, one disadvantage of operating power supplies at higher frequencies relates to a corresponding increase in core power losses that often limit the throughput power and result in an overheating of the core. Thus, improving the material properties of ferrite materials should also take into account effects on power losses.
Numerous attempts have been made to improve the chemical formulations of ferrite compositions, or the process conditions in which these compositions are sintered, in order to improve their material properties and allow these materials to operate at higher temperatures and higher frequencies with limited power losses. Some of these attempts are disclosed in U.S. Pat. Nos. 3,415,751, 3,481,876, 3,652,416, 3,769,219, 5,143,638, 5,368,763, 5,518,642, and 5,846,448. These patents disclose the use of various amounts and combinations of Fe2O3, MnO, and ZnO as major components, and one or more of Nb2O5, CaO, SiO2, V2O5, ZrO2, Al2O3, SnO2, CuO, Co3O4, TiO2, Co2O3, Li2O, Sb2O3, Ta2O5, for example, as minor components, at various processing conditions, such as sintering temperatures and pressures, that are said to provide improved properties to the ferrite material. One such objective of these attempts is to enhance the resistivity of the ferrite material by improving the grain boundary resistivity and the resistivity of the ferrite grains themselves. For example, along with the major components, prior art compositions for high frequency applications have included relatively large amounts of Co3O4, SnO2, TiO2, CaO, and the like, or combinations thereof, as minor components, in order to achieve certain material properties and characteristics.
In particular, a common approach to reduce powder losses is to increase the resistivity of the ferrite material in order to reduce eddy current losses at high frequencies. The various auxiliary additives, discussed above, in combination with Fe2O3, MnO, and ZnO have been investigated to achieve this objective. For example, one known composition that is used that is said to improve high frequency losses at frequencies up to 5 MHz includes 55-59 mol % Fe2O3, 35-42 mol % MnO, and 6 mol % or less of ZnO, with additions of 0.050-0.300 wt % CaO, 0.005-0.050 wt % SiO2, and 0.010-0.200 wt % of one or more of the following: ZrO2, Ta2O5, MoO3, In2O3, Sb2O3, and Bi2O3. Grain size of 2-5 μm is preferred in the final sintered body. Compositions outside of these ranges were are said to have higher power losses and lower minimum power loss temperatures. Representative examples of these compositions are shown in Tables 1 and 2, as comparative examples 20-22.
However, it has been found that the prior art materials are difficult to sinter and achieve consistent material properties because of their sensitivity to firing conditions. Thus, there is a continued need to provide ferrite compositions having improved and consistent material properties, such as high temperature stability and low power loss when used at relatively high frequencies, that allow for improvements in the manufacture and performance of high frequency related compounds that incorporate these materials.